Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode on both faces of the membrane-electrolyte.
The MEA generally comprises porous conductive materials, also known as gas diffusion media, which form the anode and cathode electrode layers. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and hydrogen cations. The electrons are conducted from the anode to the cathode through an electrical circuit disposed therebetween. Simultaneously, the hydrogen cations pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the hydrogen cations to form water as a reaction product.
The MEA is typically interposed between a pair of electrically conductive contact elements or bipolar plates to complete a single PEM fuel cell. Bipolar plates serve as current collectors for the anode and cathode, and have appropriate flow channels and openings formed therein for distributing the fuel cell's gaseous reactants (i.e., the H2 & O2/air) over the surfaces of the respective electrodes. Bipolar plates can be assembled by bonding together two unipolar plates having the flow distribution fields formed thereon. Typically, bipolar plates also include inlet and outlet headers which, when aligned in a fuel cell stack, form internal supply and exhaust manifolds for directing the fuel cell's gaseous reactants and liquid coolant to and from, respectively, a plurality of anodes and cathodes.
As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity to maintain an ionic resistance across the membrane within a desired range to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. Typically, the moisture is forced along the flow channels by the pressure of a gaseous reactant, with this pressure being a primary mechanism for water removal from the flow channels. However, if the pressure is not sufficient, water can accumulate in a phenomenon known as stagnation. Stagnant water can block flow channels and reduce the overall efficiency of the fuel cell. The accumulation of water can also lead to a higher rate of corrosion of the diffusion media and a poorer durability under freezing conditions. A high degree of water accumulation or stagnation can lead to fuel cell failure.
In view of the potential for water stagnation, pressure differentials between the supply manifolds and the exhaust manifolds and between adjacent flow channels or segments of the same flow channel are of considerable importance in designing a fuel cell. Along a flowfield from a reactant inlet to an outlet, partial pressures of the gaseous reactants are reduced as the reactants are consumed in a fuel cell reaction. On an anode flowfield, in particular, the pressure differential between the supply and exhaust manifolds is especially problematic due to consumption of hydrogen that occurs during fuel cell operation. Moreover, hydrogen used on the anode is less dense than O2/air and the stoichiometry on the anode is lower than on the cathode, both of which further hinder water removal on the anode flowfield.
Minimizing water stagnation has been possible, for example, by purging the channels periodically with the reactant gas at a higher flow rate or by having generally higher reactant recirculation rates. However, on the cathode of the MEA, this increases the parasitic power applied to the air compressor and reduces overall system efficiency. Additionally, the use of hydrogen as a purge gas on the anode of the MEA is not desirable for the reasons described above. The use of hydrogen as a purge gas on the anode of the MEA can lead to reduced economy, poorer system efficiency, and increased system complexity.
A reduction in accumulated water in channels can also be accomplished by lessening inlet humidification. However, it is desirable to provide at least some relative humidity in the anode and cathode reactants to hydrate the fuel cell membranes. Dry inlet gas has a desiccating effect on the membrane-electrolyte and can increase a fuel cell's ionic resistance. This method also negatively affects the long-term durability of the membrane-electrolyte.
There is a continuing need for a bipolar plate having a flowfield that militates against water stagnation in flow channels, particularly in anode flow channels. Desirably, the flowfield also achieves an optimized current density, reduces corrosion of diffusion media, and maximizes stability and freeze capability of the fuel cell during operation thereof.